1. Field of the Invention
The present invention relates in general to fluid control valves and in particular to rotary plug valves. Still more particularly, the present invention relates to rotary plug valves with seals which surround the fluid seal connection between the valve body and the plug member.
2. Description of the Prior Art
Rotary plug valves are well known in the valve art. A rotary plug valve includes a valve body with a fluid passageway therethrough. Fluid flow through the passageway is controlled by a rotary plug member which extends across the passageway and rotates between a closed position in which the plug member obstructs and seals the fluid passageway against fluid flow and an open position in which a fluid path in the plug member is connected to allow fluid flow through the fluid passageway of the valve body.
In rotary plug valves as in all valves it is necessary to provide seals to protect against leakage. These seals usually include o-ring elastomeric seals which extend around the cylinder of the plug member on opposite sides of the fluid path which extends through the plug member. These seals resiliently bear on the valve body and the plug member to seal the leak path necessarily formed to allow the plug member to rotate in the valve body between the open and closed position. This leak path would otherwise extend to the exterior of the valve body.
In some rotary plug valves, an additional elastomeric seal is provided between the plug member and valve body, which seal surrounds the valve body inlet whenb the plug member is closed. The purpose of this seal is to seal the leak path from the inlet to the outlet of the valve body. It is in addition to the sealing action of the metal to metal contact between the plug member and valve body when the plug member is in the closed position. This seal also seals the leak path to the exterior of the valve body.
Because of the configuration of the valve body and plug member, the inlet-surrounding seal must be carried by the plug member. This means that the plug member moves this seal across the inlet opening of the valve body as the plug member is rotated between the open and closed positions. This causes the seal to be pinched between the metal parts of the valve body and plug member. This can result in cutting or tearing of the seal which can destroy or reduce its effectiveness.
In some applications it is necessary to cycle a rotary plug valve at high differential pressures. Under these conditions it is possible to blow out the inlet-surrounding seal. This problem arises in part because the inlet-surrounding seals must generally have a durometer of less than about 80 in order to be sufficiently resilient to bear against the valve body with sufficient pressure to seal.
The inlet-surrounding seal most commonly used is an elastomeric o-ring. This o-ring is held in a groove machined in the exterior of the cylinder which forms the plug member. Machining this groove in the form of a ring on an exterior arc of the cylinder is obviously a difficult process because the groove is not disposed in a plane and can not be formed by a tool which rotates about the axis of the groove.
Assembly of the o-ring, inlet-surrounding seal is also difficult. The o-ring has a naturally planar shape and must be folded and held in the cylindrical arc groove of the plug member during the assembly process. The plug member is machined to fit very closely within the valve body and this further increases the difficulty of assembly.
In order to prevent pinching of the o-ring, inlet-surrounding seal as it moves across the inlet opening of the valve body it is necessary to provide a complex chamfer on the inlet open edges. This chamfer is described in U.S. Pat. No. 4,262,880. Since the inlet opening edges are located inside the valve body this chamfer also requires a difficult machining process.